Blender 2.76 AO bake partial black spots
Possible cause n°1

Some of your top faces normals are probably pointing inward, so is taking in account the inside of the pumpking and not the outside in the face-proximity calculation.



I would firtstly try to Recalculate Normals by selecting all the faces with A while in Edit mode and then press Ctrl+N.

You can display the normal vector of each face with the option you can see in the Property panel, in the Mesh Display section.

Possible cause n°2

One of your renderable objects in the scene is partially overlapped with some faces of the object you are trying to bake. Baking process takes into account also the objects in the neighborhood.